1. Field of the Invention
The present invention relates to an image processing system with a data storage unit capable of the high speed transfer of image data, such as gradation data containing color information read by a scanner.
2. Discussion of the Related Art
FIG. 5 shows a block diagram of a conventional digital image processing system.
In a digital copying machine, an image input terminal 31 (hereafter "IIT") uses a CCD line sensor to read an image on a document, which may be fed by an auto document feeder 30 (hereafter "ADF") and converts an analog video signal into a digital signal. An image processing unit 32 (referred to as an IPS) then corrects, edits, and converts the digital signal. Finally, an image output terminal 33 (hereafter "IOT") turns pixels on and off according to the image data signal, thereby to form a mesh-dot image. Alternatively, in the case of gradation image data, the IOT 33 controls the turned on time. In this way, the image on the document is reproduced.
In such a digital image processing system, the image data read by the IIT 31 is transferred to the IPS 32. In the IPS 32, the image data is temporarily stored, appropriately processed, and output to the IOT 33. A plus 1 color copying machine, which produces a copy of a monochromatic image plus one color, may use a data structure in which one pixel is expressed by three bits, two bits for gradation and one bit for a color flag. In the copying machine, parallel data representing one pixel consisting of gradation bits D.sub.0 and D.sub.1, and a color flag D.sub.2 are serially transferred every clock signal (see FIG. 6) by using a 3-bit wide bus line in combination with a clock line.
In such an arrangement, high speed image data transfer requires a corresponding increase in data transfer speed. For example, where A4 paper is laterally fed with a resolution of 400 dpi (dots per inch), an output of 40 ppm (pages per min.), and a paper movement speed of 220 mm/sec., the transfer clock frequency is required to be approximately 16 MHz.
As described above, conventional image processing systems transfer digital image data from the scanner to the image processing unit and transfer processed image data to the output terminal by simply transferring one pixel every clock signal by using a one-pixel data width bus. Therefore, the transfer clock frequency must proportionally be increased to read the image by the scanner and to print the image by the output terminal at high speed. This undesirably results in the need for expensive, high speed semiconductor devices, such as devices employing ECL technology, and further increases the generation of electric noise.